Method for removal of silver from a copper chloride solution

ABSTRACT

The invention relates to a method for removing silver from a chloride solution in a copper recovery process. According to this method, silver is removed using fine-grained copper powder and mercury. Silver removal occurs in at least two stages and mercury is fed into the solution at a certain molar ratio with regard to the silver in the solution.

The invention relates to a method for removing silver from a copperchloride solution in a copper recovery process. According to thismethod, silver is removed using fine-grained copper powder and mercury.Silver removal occurs in at least two stages and mercury is fed into thesolution at a certain molar ratio with regard to the silver in thesolution.

U.S. Pat. No. 6,007,600 describes a method for the hydrometallurgicalproduction of copper from copper-containing raw materials such as coppersulphide concentrate. According to the method the raw material isleached counter-currently with a strong sodium chloride-copper chloridesolution in several stages to form a monovalent copper(I) chloridesolution. Since both some divalent copper chloride and impuritiescomposed of other metals always remain in the solution, reduction of thedivalent copper and solution purification are performed. The purecuprous chloride solution is precipitated with sodium hydroxide tocopper oxidule and the oxidule is reduced further to elemental copper.The sodium chloride solution generated during copper oxiduleprecipitation is treated further in chlor-alkali electrolysis, and thechlorine gas and/or chloride solution obtained from this is used as araw material for leaching, the sodium hydroxide formed in electrolysisfor oxidule precipitation and the hydrogen for the reduction of copperto elemental copper. U.S. Pat. No. 6,007,600 focuses on the copperrecovery method as a whole, but silver recovery for example is notdescribed in detail.

When copper raw material is dissolved so that the copper in the chloridesolution is in monovalent form, it means that silver has also dissolved.Since the properties of the metals are near each other, separation byprecipitation with copper powder alone is not enough to provide asufficiently good end result but silver recovery requires the use ofother methods too.

According to the method in the above-mentioned U.S. Pat. No. 6,007,600copper oxidule is precipitated from the cuprous chloride solution usingsodium hydroxide. In copper oxidule precipitation, silver isprecipitated from the solution with the copper. Since the intention isto produce LME A grade copper in this process, it is important that thelevel of silver in the cuprous chloride solution corresponds to thepurity requirements of LME A grade copper. For example, the permissibleamount of silver in LME A grade cathode copper is <25 ppm (BS6017:1981). If there is for instance 60 g/L of copper in the copperchloride solution, then the amount of silver in the solution must beless than 1.5 mg/L in order to attain the required LME grade.

U.S. Pat. No. 5,487,819 describes the method developed by Intec Ltd forthe hydrometallurgical production of copper from a copper-containing rawmaterial such as copper sulphide concentrate. According to this method,the raw material is leached in a countercurrent leaching with a sodiumchloride-copper chloride solution in several stages to form a copper(I)chloride solution. The solution formed also undergoes silver removal.First the cuprous chloride solution is passed over elemental copper inorder to precipitate silver on the surface of the copper. Then thesolution is routed to the electrowinning cell, which is equipped with amixer. The cell may contain a copper anode, which is surrounded by acylindrical titanium wire gauze cathode, or the electrodes may be formedof granular or briquette copper in a titanium basket. Mercury in ionicform is fed to the solution and a Cu/Hg/Ag amalgam is formed on thecathode. The amalgam is dissolved in a strong solution of cupricchloride, which breaks the amalgam down into cupric and mercury ions.When the solution is diluted, silver chloride is precipitated and isthen treated thermally to produce metallic silver.

An article entitled “Intec Copper Sustainable Processing”, 21 Dec. 2001,is available on Intec Ltd's website which describes the present silverremoval in the copper recovery process. According to the article, theremoval of silver from the copper chloride solution going to copperelectrowinning is performed by adding soluble mercury and aluminium tothe pregnant solution. Aluminium forms a ‘copper sponge’ with the copperin the solution, which has a large surface area, enabling the silver tobe removed galvanically from the solution as an amalgam. The amalgam istreated to produce soluble mercury for recycle to the beginning of thecircuit. The silver is recovered as silver bullion.

U.S. Pat. No. 4,124,379 describes a method for the recovery of copperfrom concentrates containing silver and iron using chloride leaching. Ina silver removal column there is formed of an amalgam of mercury andsome other metal such as copper, iron or zinc. Iron and zinc alone wouldreduce the monovalent copper completely from the solution, but when theyform and amalgam with mercury, the copper is precipitated only a little.Copper is the preferred metal and can be used in granular form, which iscovered with mercury. The cuprous chloride solution is fed into thecolumn and brought in contact with the amalgam, and the metal in theamalgam replaces the silver in the solution. Silver is recovered in asilver recovery circuit, where mercury distillation takes place.

The silver electrolysis with titanium wire-gauze cathodes described inU.S. Pat. No. 5,487,819 appears complicated to implement. In the morerecent version of the process electrolysis has been replaced by theaddition of mercury and aluminium to the solution. However, the aim inproduction of pure copper is to avoid introducing all new ions such asthe aluminium ion into the solution, since they generally also requiretheir own removal process. U.S. Pat. No. 4,124,379 has two stages: thecoating of copper (or some other metal) with mercury and thedistillation of mercury, which are not recommended these days forreasons of environmental hygiene.

Now a method has been developed for the removal of silver from a cuprouschloride solution in the hydrometallurgical recovery process of copper.The method is simpler than those described above and does not requirethe addition of extra substances to the cuprous chloride solution otherthan soluble mercury and copper powder.

It is characteristic of the method of this invention that theprecipitation of the silver from the cuprous chloride solution occurs inat least two stages using soluble mercury and fine copper powder. In thesilver recovery stages there is some fine-grained copper in theprecipitation stage reactors, which deposits the silver from thesolution. This reaction does not entirely reach completion, as thecopper also acts as a precipitation surface in the amalgam precipitationthat occurs with mercury. In the first amalgam precipitation stage themolar ratio of the mercury to be fed into the solution to the silver inthe solution is adjusted to between 0.5-2 to one and in the second stagethe mercury: silver molar ratio is adjusted to a minimum of 2 to one.Any mercury left in the cuprous chloride solution is precipitated outwith fine copper powder so that the solution going for further treatmentis mercury-free. The deposit formed in precipitation and the remainingfine-grained copper in it is recycled countercurrently to the directionof the solution in the silver removal stages. The precipitatedsilver-amalgam is treated to leach the mercury and recycle it back tothe silver precipitation stages while the silver is deposited as silverchloride.

In the hydrometallurgical recovery of copper based on chloride leachingthe first stage is usually the leaching of a sulphidic copperconcentrate, where the copper in the subsequent solution is mainlymonovalent. In these circumstances silver also dissolves in monovalentform. Whether the further treatment of the solution is electrowinning orcopper oxidule precipitation, it is beneficial that all the copper inthe solution is monovalent. For this reason after leaching the divalentcopper of the concentrate in the solution is removed either by reductionor precipitation. The next process stage is the removal of impurities(other metals) and silver removal can be seen as a part of this stage.The precipitation of silver from the cuprous chloride solution as anamalgam in accordance with this invention is simple, in that specialconditions are not required. In fact it can be carried out at thetemperature and pH the solution has when it comes from the precedingprocess stage. The temperature of the solution from the divalent copperremoval stage is in the range of 50-70° C. and the pH 1-5. The reactorsused in the different stages of silver removal are mixing reactors. Thestage may include one or several reactors, although in the descriptionof the invention we speak for simplicity of only one reactor per stage.Silver removal can be performed either as a batch or a continuousprocess. In particular, it is preferable to arrange the flow of solutionfrom one stage to the next continuously.

The first stage of silver removal is preferably a precipitate stageusing only fine-grained copper. In this way using only copperprecipitate the silver level in the solution can drop to around 30 mg/land at the same time the use of mercury in subsequent stages isminimized. The precipitated metallic silver can be kept in the reactoruntil all the copper has dissolved and the pure (in practice over 90%)silver powder can be recovered from the reactor. Copper precipitationoccurs according to the following reaction:Cu+Ag⁺→Ag+Cu⁺  (1)If the amount of silver in the raw material is small, for instance lessthan 30 mg/l, the silver precipitation stage performed with copperpowder only may be omitted completely and the precipitation stagesperformed using mercury only may be used instead.

The cuprous chloride solution is fed to the second stage of silverremoval, which takes place with mercury. This can also be called thefirst amalgam precipitation stage. The reactor contains copper,including also copper amalgam, which has been moved there from a latersilver removal stage. An amount of soluble mercury is fed to thesolution with the molar ratio of between 0.5 and 2, preferably 1, to theamount of silver present in the cuprous chloride solution. The majorityof the silver in the solution precipitates in this stage as a silveramalgam. The reactions can be described for example as follows:2Cu+Hg⁺→CuHg+Cu⁺  (2)CuHg+Ag⁺→HgAg+Cu⁺  (3)

The reactions show that when the silver and mercury precipitate from thesolution as a silver amalgam, the copper dissolves at the same time. Theprecipitate is removed from the reactor for mercury leaching and silverrecovery.

The cuprous chloride solution is fed to the third silver removal stage,which may also be termed the second amalgam precipitation stage. Themolar ratio of the mercury fed to this stage to the silver in thesolution is at least two to one, preferably five to one. Since there isonly a few milligrams of silver left dissolved in the cuprous chloridesolution, less than ten per cent, the amount of mercury required in thisstage is however smaller than the amount fed to the first stage.Additionally in this stage there is some fine-grained copper in thereactor, fed there from the mercury removal stage. Mercury isprecipitated from the solution forming a copper amalgam on the surfaceof the copper, onto which silver is precipitated as a silver amalgam inaccordance with reactions (2) and (3). The precipitate is conveyed tothe first amalgam precipitation stage. After precipitate, the silvercontent of the cuprous chloride solution removed from the third stagehas fallen to such a level that the amount of silver in the finalproduct is less than that demanded for the LME grade.

In order for the cuprous chloride solution to be mercury-free, a furthermercury removal is performed on the solution in the mercury removalstage. Mercury is removed using fine-grained copper and the amount of Cupowder to be fed is in the range of 100 g/l when its particle size isbelow 200 μm. The copper used in the precipitation stages may becoarser, but in that case the amount used is greater, because theprecipitation surface decreases as the particle size grows. The solidmaterial that settles on the reactor floor is moved countercurrently tothe solution i.e. the solids obtained from the mercury removal stage arerecycled to the third silver removal stage and from there on to thesecond stage, where they are removed for silver and mercury separation.

The deposit removed from the second silver removal stage (the firstamalgam precipitation stage) contains mainly a silver amalgam, whichincludes a little copper. The deposit is leached into a dilute chloridesolution by oxidation. The oxidant may be for instance hydrogen peroxideH₂O₂, oxygen O₂ or sodium hypochlorite NaOCl. During leaching mercurydissolves and is fed as a mercury chloride solution back to theprecipitation stages. Silver precipitates in these conditions as silverchloride and is routed for the desired further treatment for recovery asmetallic silver. The Hg chloride content of the solution obtained fromthe leaching stage is adjusted to get the correct molar ratio beforebeing fed to the precipitation stages.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described further with the aid of the attached diagram,where

FIG. 1 is a flowsheet of one method in accordance with the invention.

According to the flowsheet in FIG. 1 a cuprous chloride solution is fedto the first silver removal stage I, where part of the silver in thesolution is removed by copper powder precipitation alone. The amount ofcopper powder fed to stage I is in the range of 100 g/L, when theparticle size is less than 200 μm. The reactor used in this stage is amixing reactor, from which both the solution and the metallic silvermixed in it can be fed to the subsequent stage or the silver separatedfrom the bottom of the reactor by dissolving the copper (not shown inthe diagram).

The cuprous chloride solution from stage I is fed to stage II, which isthe first amalgam precipitation stage. Soluble mercury is also added inthe form of mercury chloride for example. The amount of mercury to befed is in a molar ratio of 0.5-2:1 of the amount of silver in thesolution. In the final silver removal stage III the precipitated silveramalgam and the undissolved copper powder are fed to the precipitationstage II, countercurrently to the cuprous chloride solution. Over 90% ofthe silver still in the solution is precipitated in the second stage,and the silver amalgam is fed to the silver separation stage IV.

In separation stage IV the silver amalgam is leached in a dilutechloride solution by oxidizing the solution. Oxidation can take placefor instance using sodium hypochlorite. As a result of leaching themercury dissolves as mercury chloride and the silver is precipitated assilver chloride. The mercury chloride solution is fed back to stages IIand III. If any undissolved copper has been remained in the amalgamdeposit when the deposit is routed to the oxidation/leaching stage, theentry of copper in the mercury chloride solution does not harm theprocess.

The third silver removal stage III i.e. the second amalgam precipitationstage functions like the first one, but now the molar ratio of themercury to be fed to the solution to that of the silver in the solutionis determined so that there are at least 2, preferably 5, moles ofmercury to each mole of silver. The molar ratio may be anywhere between2-10. The precipitate from the final stage V of silver removal is alsoconveyed to this stage. The silver content of the solution leaving thisstage is less than 1 mg/L, which corresponds to a content of <25 ppm inthe finished product. The silver amalgam precipitated from the solutionand any undissolved copper powder are conveyed on to the second silverremoval stage.

So that no mercury remains in the cuprous chloride solution, mercury isremoved from the solution in mercury removal stage V by addingfine-grained copper powder to the solution. The amount of copper powderto be fed is around 100 g/l, when the particle size of the powder isbelow 200 μm. The mercury in the solution is precipitated on the surfaceof the copper in accordance with reaction (2) so that in practice thereis no mercury in the solution exiting this stage. The precipitate isconveyed to the second amalgam removal stage. After the removal of themercury the cuprous chloride solution is routed to the other stages ofsolution purification.

EXAMPLE 1

The removal of silver from a cuprous chloride solution was studied incontinuous laboratory pilot tests. Silver removal was done in threestages in mixing reactors connected in series. A batch of fine-grainedcopper powder with an average particle size of 100 μm was placed in thereactors. The effective volume of the reactors was 1.5 litres. The feedsolution was a concentrated chloride solution of monovalent copper witha copper content of 60 g/L and a sodium chloride content of about 280g/L. The solution flow was 1.5 L/h and the temperature 60° C. The silvercontent of the feed solution was 110 mg/L and the pH 3. The target wasto reduce the silver content of the solution to below 1 mg/L.

In the first stage silver was removed from the solution by precipitationin the mixing reactor using a batch of pure copper powder. In thisprecipitation stage the silver content of the solution fell to around 30mg/L.

The solution, with a silver content of around 30 mg/L, was routed to thesecond stage, where there was a batch of copper powder in the mixingreactor. 60 mg/L of mercury as a solution of HgCl₂ was fed continuouslyto the second stage, corresponding to a molar ratio of 1:1 with regardto the silver content of the feed solution. Silver and mercury wereprecipitated from the solution together forming an AgHg amalgamcorresponding to their feed ratio on the surface of the copperparticles. At the same time copper dissolved in the solution as Cu⁺ions. After the second stage the silver content of the solution wasabout 3 mg/L.

The solution, with a silver content of about 3 mg/L, was routed to thethird stage, where there was a batch of copper powder in the mixingreactor. 30 mg/L of mercury as a solution of HgCl₂ was also fedcontinuously to the third stage, corresponding to a molar ratio of 5:1with regard to the silver content of the feed solution. Mercury wasprecipitated from the solution forming a layer of CuHg amalgam on theparticle surface. The CuHg amalgam formed deposited the silver from thesolution. After the third silver removal stage the silver content of thesolution was as targeted below 1 mg/L.

In all three silver removal stages it was possible to utilize the copperbatches in the reactors in the precipitation reactions almostcompletely. Thus in theory the feed of solution could continue as longas copper remained in the reactors. In practice at the end of the testsless than 5% copper remained in the solids in the reactors.

The HgCl₂ solution used for the mercury addition was prepared byleaching a deposit containing mercury, silver and copper. The deposit isformed in the second silver removal stage, when silver and mercury areprecipitated on the surface of copper powder and the copper dissolves atthe same time. This deposit is leached by oxidizing into a dilutechloride solution, whereby the silver is recovered as a poorly solublesilver chloride (AgCl) and the mercury can be recycled as an HgCl₂solution to the silver removal precipitation stage. If the depositcontains copper, it dissolves and ends up in solution with the mercuryand further on in the silver removal precipitation stage.

50 g deposit, with a composition of 65% Hg, 25% Ag and 10% Cu wasleached by oxidizing into 1 litre of 1 M HCl solution at a temperatureof 80° C. The oxidant used was a solution of NaOCl, which maintained ahigh oxidation potential, over +800 mV (vs. AgCl/Ag). The mercury andthe copper dissolved into the solution as divalent ions. The silver andthe chloride formed silver chloride, which is poorly soluble in a dilutechloride solution. The AgCl deposit was removed from the solution.

After the three-stage silver removal series a fourth mixing reactor wasconnected, with a batch of copper powder in the reactor. The copperdeposited the mercury left in the solution from the previous stages, andafter mercury removal the Hg content of the solution was less than 0.2mg/L.

1. A method for the removal of silver from a cuprous chloride solutionin a copper recovery process, comprising removing, in at least twostages, silver from the cuprous chloride solution with soluble mercury,wherein fine-grained copper powder is fed countercurrently to thecuprous chloride solution, the method comprising: feeding mercury intothe cuprous chloride solution at preselected stages in a preselectedmolar ratio to the silver in the solution; precipitating silver amalgamonto the surface of fine-grained copper powder; removing theprecipitated silver amalgam from the cuprous chloride solution for theseparation of mercury and silver; recycling soluble mercury back tosilver removal; and treating the removed precipitated silver amalgam forthe recovery of silver; wherein the treating step comprises leaching theprecipitated silver amalgam into a dilute chloride solution using anoxidant, whereby the mercury dissolves as mercury chloride and thesilver precipitates as silver chloride.
 2. The method according to claim1, wherein the molar ratio of mercury to silver in a first silverremoval stage is 0.5-2.
 3. The method according to claim 1, wherein themolar ratio of mercury to silver in a second silver removal stage is atleast
 2. 4. The method according to claim 3, wherein the molar ratio ofmercury to silver in the second silver removal stage is between 2-10. 5.The method according to claim 1, wherein the fine-grained copper powderhas a particle size of less than 200 μm.
 6. The method according toclaim 5, wherein the amount of fine-grained copper powder beingcountercurrently fed is about 100 g/L based on the cuprous solution. 7.The method according claim 1, further comprising feeding thefine-grained copper powder countercurrently to a mercury removal stageafter the at least two silver removal stages.
 8. The method according toclaim 1, wherein the oxidant is sodium hypochlorite.
 9. The methodaccording to claim 1, wherein the oxidant is hydrogen peroxide.
 10. Themethod according to claim 1, wherein the oxidant is oxygen.
 11. Themethod according to claim 1, further comprising routing the dissolvedmercury chloride back to the leaching step.
 12. The method according toclaim 1, further comprising routing the silver chloride to a silverrecovery step.
 13. The method according to claim 1, wherein the chloridecontent of the dilute chloride solution in the leaching step is at least200 g/L.
 14. The method according to claim 1, wherein the cuprouschloride solution comprises 30-100 g/L of monovalent copper.
 15. Themethod according to claim 1, wherein the cuprous chloride solution haspH value of 1-5 in the precipitating and removing steps.
 16. The methodaccording to claim 1, further comprising removing silver from thecuprous chloride solution using fine-grained copper powder in a stageprior to the at least two silver removal stages.
 17. The methodaccording to claim 16, wherein the fine-grained copper powder has aparticle size of less than 200 μm.
 18. The method according to claim 17,wherein the amount of fine-grained copper powder being countercurrentlyfed is in the range of 100 g/L.